The Rankine cycle is a power generation cycle that converts thermal energy to mechanical work. The Rankine cycle is typically used in heat engines, and accomplishes the above conversion by bringing a working substance from a higher temperature state to a lower temperature state. The classical Rankine cycle is the fundamental thermodynamic process underlying the operation of a steam engine.
In the Rankine cycle a heat “source” generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the “working body” of the engine while transferring heat to the colder “sink” until the working substance reaches the lower temperature state. During this process, some of the thermal energy is converted into work by exploiting the properties of the working substance. The heat is supplied externally to the working substance in a closed loop, wherein the working substance is a fluid that has a non-zero heat capacity, which may be either a gas or a liquid, such as water. The efficiency of the Rankine cycle is usually limited by the working fluid.
The Rankine cycle typically employs individual subsystems, such as a condenser, a fluid pump, a heat exchanger such as a boiler, and an expander turbine. The pump is frequently used to pressurize the working fluid that is received from the condenser as a liquid rather than a gas. Typically, all of the energy is lost in pumping the working fluid through the complete cycle, as is most of the energy of vaporization of the working fluid in the boiler. This energy is thus lost to the cycle mainly because the condensation that can take place in the turbine is limited to about 10% in order to minimize erosion of the turbine blades, while the vaporization energy is rejected from the cycle through the condenser. On the other hand, the pumping of the working fluid through the cycle as a liquid requires a relatively small fraction of the energy needed to transport the fluid as compared to compressing the fluid as a gas in a compressor.
A variation of the classical Rankine cycle is the Organic Rankine cycle (ORC), which is named for its use of an organic, high molecular mass fluid, with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. As such, in place of water and steam of the classical Rankine cycle, the working fluid in the ORC may be a solvent, such as n-pentane or toluene. The ORC working fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds, etc. The low-temperature heat may then be converted into useful work, which may in turn be converted into electricity.